In the fabrication of semiconductor devices, various processing steps are started with a photolithographic process to first define a circuit on the wafer. For instance, in a modern memory device, multiple layers of metal conductors are required for providing a multi-layer metal interconnect structure. As the number of layers of metal interconnects increase, while the device geometry continuously decreases to allow more densely packed circuits, the photolithographic process required to define patterns of circuits becomes more complicated and difficult to carry out.
After a process for forming metal vias or lines in an insulating layer is completed, a photoresist layer must be stripped. As a result, it is necessary to subject the wafer to a wet etching process for removing the photoresist layer. For instance, a wet stripping process can be implemented after a photoresist dry stripping process by utilizing a wet etchant such as ACT® 690C or EKC® 265 in order to remove the photoresist layers. The ACT® 690C is a mixture of DMSO (dimethyl-sulphur-oxide), MEA (mono-ethyl-amine) and catechol, while EKC® 265 is a mixture containing HDA (hydroxy-amine). The conventional wet dip process requires a special buffer solvent treatment step in order to avoid or minimize metal corrosion problems in the circuits already formed on the wafer surface.
Wet etching is the more frequently used technique for stripping photoresist films from silicon wafers where the complete removal of resist images without adversely affecting the wafer surface is desired. The resist layer or images should be completely removed without leaving any residues, including contaminant particles that may have been present in the resist. The underlying surface of the photoresist layer should not be adversely affected, for instance, accidental etching of the metal or oxide surface should be avoided. Liquid etchant strippers should produce a reasonable yield in order to prevent redeposition of dissolved resist on the wafers. The etchant should completely dissolve the photoresist layer in a chemical reaction, and not just lifting or peeling so as to prevent redeposition. It is also desirable that the etching or stripping time be reasonably short in order to permit high wafer throughput.
Other wet etchants such as sulfuric acid (H2SO4) and mixture of H2SO4 with other oxidizing agents such as hydrogen peroxide (H2O2) may also be used in stripping photoresist or in cleaning a wafer surface after the photoresist has been stripped by other means. For instance, a mixture may be seven parts H2SO4 to three parts 30% H2O2, or a mixture of 88% sulfuric acid and 12% nitric acid. Wafers to be stripped can be immersed in the mixture at a temperature between about 100° C. and about 150° C. for 5˜10 minutes and then subjected to a thorough rinse of deionized water and dried in dry nitrogen. This type of inorganic resist strippers, such as the sulfuric acid mixtures, is very effective in the residual-free removal of highly post-baked resist. They are more effective than organic strippers and the longer the immersion time, a cleaner and more residue-free wafer surface can be obtained.
The photoresist dry stripping process is normally carried out by a plasma etching or sometime known as a “plasma ashing” process which is effective in ashing organic photoresist layers. The plasma ashing process is an isotropic etch process for organic photoresist in an oxygen glow discharge where atomic oxygen is produced. Oxygen atoms react with organic photoresist material to form volatile products such as CO1 CO2 and H2O. A barrel-type reactor is frequently used for the plasma ashing processing. For instance, a batch of wafers such as 25 wafers coated with 1 μm photoresist layer can be processed in less than 10 min.
When the plasma ashing method is used to remove photoresist layers on a wafer surface, the wafer is frequently ground on the backside and therefore, a bare silicon surface is exposed when the wafer is loaded into the plasma etch chamber. During the oxygen plasma ashing process, not only the top side of the wafer is etched, the peripheral edge of the backside of the wafer is also attacked by the oxygen plasma. As a result, a sawtooth shaped pattern of silicon oxide is formed along the peripheral edge of the wafer on the backside. This is shown in FIG. 1. For wafer 30, a sawtooth shaped peripheral area 32 where the bare silicon exposed by the backside grinding process is oxidized by the oxygen plasma forming silicon oxide. The surface oxidation of the bare silicon is mainly caused by oxygen plasma that flows into the shallow trench lines 34 that are provided on the surface of the heater plate in the plasma etcher for holding the wafer. At near the three ejector pin holes 36, the oxygen plasma also tend to leak in and cause oxidation of the bare silicon surface near the holes. The silicon oxide layer formed along the peripheral edge of the wafer backside makes the backside surface uneven and furthermore, the thickness of the wafer uneven. Such uneven thickness of the wafer may cause additional stress problems. When another backside grinding process is attempted to even out the backside surface, the wafer may break during the grinding process and cause a catastrophic loss.
It is therefore an object of the present invention to provide a method for plasma etching a wafer after a backside grinding process that does not have the drawbacks or shortcomings of the conventional plasma etching method.
It is another object of the present invention to provide a method for plasma etching a wafer after a backside grinding process exposing a bare silicon surface that does not oxidize the backside of the wafer.
It is a further object of the present invention to provide a method for plasma etching a wafer after a backside grinding process that does not cause the formation of silicon oxide on the backside of the wafer.
It is another further object of the present invention to provide a method for plasma etching a wafer after a backside grinding process by first oxidizing the wafer before the plasma etching process is carried out.
It is still another object of the present invention to provide a method for plasma etching a wafer after a backside grinding process by first placing the wafer in an oxidation furnace for growing a uniform layer of silicon oxide on the backside of the wafer prior to the plasma etching process.
It is yet another object of the present invention to provide a method for plasma etching a wafer after a backside grinding process by first growing a silicon oxide layer that is at least 50 Å thick on the backside of the wafer.
It is still another further object of the present invention to provide a method for plasma etching a photoresist layer on a wafer after a backside grinding process is conducted wherein the oxygen plasma does not form a silicon oxide layer on the backside of the wafer.